This invention relates to a drive for a hold down assembly for use in the manufacture of cans. In particular, but not exclusively, it relates to a drive for a blank holder which holds a can blank against a redraw die.
Hold down mechanisms such as redraw sleeves and blanking punches are known. Typically, a lever is held against cam profiles on the crank. The lever drives a pair of push rods to drive a crosshead which, in turn, actuates a blank holder. This combination of push rods and cam actuation moves the blank holder towards a redraw die to bring the can blank, or cup, to the die. The blank holder presses the base of the cup against a flat face of the die while a punch pushes the cup into the die for redrawing.
This type of mechanism is heavy and the rotating mass on the crankshaft presents a severe load to the bodymaker main bearings. This invention seeks to reduce problems associated with this loading.
According to the present invention there is provided a hydraulic drive for a hold down apparatus in a can bodymaker, the drive comprising a fixed guide rod; a guide pod surrounding the guide rod, the pod having rear and forward end faces which together define rear and forward hydraulic chambers respectively, the chambers being separated by a seal; a first channel (A) for the passage of hydraulic fluid to and from the rear hydraulic chamber via a return stroke port; a second channel (B) for the passage of hydraulic fluid to and from the forward hydraulic chamber via a forward stroke port; whereby passage of fluid into the forward chamber drives the pod and hold down assembly connected thereto to a forward position and passage of fluid into the rear chamber forces the pod and hold down assembly to return to a back position.
By using a hydraulically powered drive, the rotating mass on the bodymaker crankshaft is dramatically reduced since the existing push rods, cam levers, redraw cams and cam followers and air bags to hold the cam followers onto the cams are no longer required. This in turn decreases the size of the bodymaker hydraulic power pack which is required for the push rods, cams and followers in known hold down apparatus. Furthermore, an increase in machine speed is possible due to the reduction in mass and subsequent reduction in system inertia which could lead to increased production.
Various knock-on effects are achieved by the use of the hydraulic drive for the hold down assembly, such as a reduction in size of power components, flywheel and other drives etc. and thereby reducing load on the bodymaker main bearings and wear.
The rear and forward end faces of the pod may typically be defined by bushings.
The hydraulic fluid may be the machine coolant which is typically already available in the factory supply. Although this may require of the order of 60 liters/minute, the bodymaker hydraulic power pack can in fact be reduced in size due to the replacement of several components as noted above. The replacement operation is possible simply by means of a retro-fit.
The forward chamber typically comprises a substantially cylindrical portion which tapers radially outwardly at its forward end whereby pressure in the hydraulic chamber is decreased at the forward end. The taper, or chamfer decreases hydraulic pressure at the forward end of the hydraulic chamber since the chamber size is increased at the fluid pressure face but limits fluid requirements in the remainder of the chamber.
The hydraulic drive may ideally include check valves for controlling initial acceleration of the guide pod and/or pressure relief valves for the avoidance of pressure spikes.
Whilst the hydraulic fluid flow may be controlled by a variety of means, ideally a rotary valve is used. The rotary valve may rotate at a speed which is less than or equal to machine speed, according to the desired machine timing.
According to a further aspect of the present invention, there is provided a method of driving a hold down apparatus in a bodymaker, the method comprising: providing a fixed guide rod; connecting the hold down apparatus to a guide pod which surrounds the guide rod and is movable along the guide rod, the pod having rear and forward end faces which define rear and forward hydraulic chambers respectively, the chambers being separated by a seal; supplying hydraulic fluid to and from the rear hydraulic chamber via a return stroke port; supplying hydraulic fluid to and from the forward hydraulic chamber via a forward stroke port; whereby supplying fluid into the forward chamber drives the pod and hold down assembly connected thereto to a forward position and supplying fluid into the rear chamber forces the pod and hold down assembly to return to a back position.
Preferably, the end faces comprise bushings for covering and/or opening the ports, and the method further comprises: accelerating movement of the pod and hold down apparatus by uncovering a port and increasing fluid flow to and from the respective chamber; or decelerating the machine stroke by covering a port and reducing fluid flow to and from the respective chamber.
Preferred embodiments of hydraulic drive will now be described, by way of example only, with reference to the drawings, in which: